1. Field of Invention
This invention relates to a method and apparatus for filtering a fluid through a porous membrane. More particularly, this invention relates to a filtration method and apparatus which creates large oscillatory fluid flow essentially parallel to a membrane surface to effect fluid flow both through the membrane and tangentially across the membrane.
2. Description of Prior Art
Membrane filtration processes have long been employed for the selective removal of contaminants from process fluid streams. For example, microfiltration membranes have been employed for the removal of bacterial contaminants from a wide variety of fluids, including beverages, parenterals and ophthalmic solutions, and for the removal of cellular debris from cell lysate streams. Ultrafiltration membranes, having a smaller pore size than microfiltration membranes, have been employed for the concentration of protein solutions as well as for the selective fractionation of proteins in solution. However, during many filtration processes, the presence of certain species can either irreversibly bind to the membrane surface or clog the membrane pores, resulting in membrane fouling that significantly decreases the rate at which fluid can permeate through the membrane structure. Similarly, the presence of certain proteins can lead to concentration polarization, which is the formation of a dense layer of highly concentrated protein near the membrane surface. This concentration polarization layer will also significantly reduce the rate at which fluid permeates through the membrane structure. Consequently, the removal of these fouling and/or concentration polarization layers can significantly improve the performance and economics of membrane filtration processes.
To minimize the formation of these layers, filtration processes have conventionally been conducted in the tangential flow filtration (TFF) mode of operation, wherein the flow of fluid is directed parallel to the membrane surface with only a fraction of the fluid being directed through the membrane. Membrane filters used in TFF processes are generally configured as flat sheet membrane cassettes where parallel membrane sheets are separated by thin, fluid-filled gaps. A fluid feed solution, typically consisting of cells, cellular debris and/or soluble proteins, is continuously recirculated at high velocities within the fluid-filled gap between the parallel membrane sheets using an external pump capable of delivering the necessary inlet pressures. The high fluid velocity serves to continually sweep the membrane surface clean of particles and proteins responsible for the formation of fouling and concentration polarization layers. As a consequence of the high transmembrane pressures created by the recirculation pump, a small fraction of the feed material is directed through the parallel membranes and is collected as permeate product.
A major limitation associated with this technology is that the external recirculation pump is responsible for creating both the high fluid velocities required for cleaning the membrane and the transmembrane pressures required for directing a portion of the fluid flow through the membrane structure. Consequently, the hydrodynamics required for cleaning the membrane surface are intimately coupled with the transmembrane pressure required for filtering the fluid material. For example, an increase in the fluid velocity across the membrane surface is accomplished by increasing the inlet pressure to the membrane device. These higher inlet pressures, while increasing the shear rates at the membrane surface, also cause more of the feed material to be directed through the membrane structure, ultimately accelerating the formation of the fouling and concentration polarization layers. In an effort to combat this phenomenon, cross flow velocity is increased, thereby increasing the upstream pressure gradient and further accelerating the fouling process. Consequently, each filtration process requires independent optimization. More importantly, highly viscous fluids and/or complex, highly fouling fluids cannot be processed at sufficiently high fluid shear rates to prevent the formation of fouling and/or concentration polarization layers.
Several techniques have been developed that attempt to address the limitations associated with conventional TFF operations. Vibrational filter systems are described in the prior art that are comprised of a constant gap spacing between two plates in which the filter elements are vibrated to prolong filter life by either minimizing cake formation or by removing existing cakes are well known. U.S. Pat. No. 3,970,564 discloses a vibration-scheme for efficiently removing pre-dried cakes from a series of stacked filter discs. U.S. Pat. No. 4,076,623 discloses a methodology in which the filter medium is reciprocated in a non-uniform fashion in a direction parallel to fluid flow in an effort to retard cake formation. U.S. Pat. No. 4,836,922 details a technique in which a series of filter cartridges are oscillated within the feed material to minimize cake formation. U.S. Pat. No. 4,872,988 describes a technique where the filter is oscillated at a prescribed frequency and displacement relative to the suspension in order to inhibit filter plugging. CH Patent 667217A5 discloses a technique in which oscillations generated by a piezoelectric ultrasound generator are transmitted to a belt filter for the subsequent removal of filter cakes.
In contrast, prior art methodologies for pulsating the fluid flow relative to the filter element, have also been disclosed in the prior art in an effort to minimize cake formation on filter devices. U.S. Pat. No. 4,886,608 describes a technique in which the vacuum downstream of a filter device is pulsed in order to remove solid material from the filter medium. DE Patent 3605065A1 discloses a similar technique in which the filtrate is pulsed in a periodic fashion to minimize cake formation. U.S. Pat. No. 3,692,178 describes a technique where the vibration of supply and exhaust air lines causes the process feed fluid to agitate within the filter housing, thereby enhancing fluid shear at the filter surface.
U.S. Pat. 5,468,844 discloses a technology consisting of a filtration channel comprised of a stationary membrane disc mounted in close proximity to a solid disc capable of being rotated at high angular velocities. The mechanical rotation of the solid disc causes the fluid between the membrane and solid disc to rotate in a direction generally parallel to the membrane surface, thereby minimizing the formation of fouling and concentration polarization layers. Flow of fluid through the membrane structure is accomplished by independently increasing the pressure upstream of membrane relative to the pressure downstream of the membrane device. However, there are two limitations associated with this technology. First, due to the circular geometry of this device, the fluid flow and shear are unevenly distributed across the plate radius, leading to a non-uniform use of the membrane surface. Second, viscous heating of the fluid caused by the overly large angular velocities of the rotating devices significantly hinder its applicability for large-scale separations.
International patent WO 97/02087 discloses a filtration technology which is comprised of disc plates that oscillate in an angular direction about a center axis. This oscillatory motion causes the fluid between the membrane discs to be moved and sheared generally in a tangential direction, effectively preventing the formation of fouling and concentration polarization layers. However, a limitation associated with this technology is that the tangential motion, velocity, and shear are highly non-uniform over the disc radius leading to a non-uniform use of the membrane surface. In order to more effectively use the membrane surface, the angular velocity of the oscillation must be increased which results in over-shearing, undesirable power consumption and heating of the fluid at the larger radii. The torsional oscillatory disc motion is supplied by a source external to the filter device and is transmitted to the device disc plates by means of a shaft. Because of the resulting increased inertia of the discs and the increasing length of the oscillating drive shaft, scale-up of the technology while maintaining efficient, uniform motion becomes impractical.
U.S. Pat. No. 4,158,629 describes the operation of a dynamic self-cleaning filter for liquid streams. In this technology, an outer resonant tube is concentrically mounted around an inner composite tube containing a filter membrane. The fluid in the annular chamber is transformed into a state of intense vaporous cavitation by causing the outer tube to resonate with the use of a sonic sinusoidal wave inducing transducer fixed to the outer tube at an antinodal point. The vaporous cavitation energy implodes the surfaces of the filter material, thereby minimizing the formation of fouling and concentration polarization layers. However, the intense energy involved with cavitation, although effective in cleaning membrane surfaces, would most likely lead to the denaturation of bioproducts sensitive to the formation of gas/liquid interfaces. In addition, cavitation upstream of polymeric membranes can conceivably damage the membrane surface.
It would be desirable to provide a method and apparatus of generating high shear forces at membrane surfaces in order to minimize fouling and concentration polarization, independent of variables controlling the feed rate and the fluid permeation rate through the membrane structure. Furthermore, it would be desirable that such a method and apparatus permit scaling up to commercially viable filtration rates. It would further be desirable to avoid cavitation that may degrade bioproduct quality and reduce membrane stability. In addition, it would be desirable to provide such a method and apparatus which improves the ease-of-use characteristics of dead-ended membrane filtration compared to conventional TFF operations.